Phase change materials ("PCMs"), also known as thermal energy storage compositions, store heat during phase transition, typically liquid/solid phase transitions. A large amount of thermal energy can be stored as latent heat of fusion during the melting of the PCM. For this reason, PCMs are often incorporated into thermal energy storage and heat exchanger apparatuses. During the operation of such an apparatus, heat from the surrounding air is transferred to the PCM as heat of transition, until the frozen PCM completely melts. Additional heat from the surrounding air can then be stored within the PCM as sensible heat. The heat stored within the PCM may be discharged from the apparatus by passing relatively cool air past the liquid PCM. The liquid PCM transfers its heat to the air stream, and thus, the temperature of the air stream is raised and the PCM is re-cooled.
Various attempts have been made to incorporate PCMs into heating and air conditioning systems, including heat pump systems, solar collection systems, and more conventional heating and air conditioning systems for homes, vehicles, and similar structures requiring heating and cooling. For example, U.S. Pat. No. 5,054,540 to Carr describes a cool storage reservoir positioned in an air duct of a vehicle or the like. Another example is the "heat battery" designed to provide "instant heating" to a vehicle cabin. (Automotive Engineering, Vol. 100, No. 2, February, 1992.)
A variety of materials may be used as PCMs. For example, water, paraffins, alcohols, and salt hydrates have notably high energy densities over temperature ranges of practical significance. Water, however, is of particular interest because it is plentiful, inexpensive, and environmentally friendly. As a heat storage material, it has good heat capacity, heat transfer properties, and an acceptable density. Additionally, the transformation of the water into ice has a heat of fusion of 80 cal/g, and it occurs at 0.degree. C.
However, the use of water as a PCM presents some difficulties. For example, conventional air conditioning units must be reconfigured to operate at the ice temperature, and, in an air heat exchanger, moisture can freeze on the cooling coil. Other problems derive from the fact that the freezing of water to ice is accompanied by approximately a 9% volume expansion. This anomalous expansion during freezing is the cause of burst water pipes in homes during cold weather, and cracked radiators/engine blocks in cars. What is needed is an aqueous PCM, which has the energy storage qualities of water but lacks the destructive freeze characteristics associated with water's transformation to ice.
According to the present invention, there is provided a method for reducing rupture failures during freezing cycles of a heat exchange/storage device utilizing an aqueous PCM composition by adding to the aqueous PCM composition a freeze-modifier comprising an non-ionic surfactant, preferably alcohol ethoxylates, and an anionic surfactant in an amount effective to stabilize, i.e. to reduce phase separation of, the composition at elevated temperatures.
Also in accordance with the present invention there is provided a water-based thermal energy storage composition capable of forming a discontinuous ice phase and of significantly maintaining the energy storage properties of water. The composition comprises about 60 to about 96.5 weight percent water, about 3.0 to about 40 weight percent of one or a mixture of water-dispersible non-ionic surfactants and an anionic surfactant in an amount sufficient to stabilize the composition at elevated temperatures.
An objective of the present invention is to modify the freeze characteristics of water, specifically the single piece characteristic of ice while significantly maintaining the energy storage capacity and freezing point of the water. With use of this invention ice forms as discrete particles, forming a kind of "slippery ice" with concomitant reduction of the volume expansion accompanying ice formation. Further, the composition in accordance with the present invention prevents the formation of a single, solid, block of ice and reduces freeze expansion.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.